Dampers and Mixing Sections: EN 1751 Leakage and Actuator Sizing

A damper is the cheapest component in an air handling unit and the one most likely to be specified by habit. It arrives on the drawing as a rectangle, it is priced by area, and the selection sheet frequently says nothing about it at all. Meanwhile it decides three things the rest of the unit is judged on: how much untreated air enters when it is closed, how much fan power it costs when it is open, and whether the actuator will still drive it after five winters of seal compression.

Two standards, two different leakages

The most common specification error in this area is conflating casing leakage with damper leakage. They are separate tests in separate standards and they fail in separate ways.

EN 1886 governs the AHU casing: classes L1 / L2 / L3 for air leakage through the box at a stated test pressure, plus filter bypass leakage class. That is the sheet metal around the component.

EN 1751 governs the damper itself, and it publishes two independent classes:

The limits are given as leakage per square metre of damper face against pressure difference, so a class is only meaningful with the test pressure quoted alongside it — and the ladder from class 0 to class 4 is steep, roughly a factor of three per step. A specification that says "low-leakage damper" without a class and a test pressure has specified nothing. Ask for the number the manufacturer measured on that product, at the differential pressure your unit will actually see across a closed blade, which on the outdoor-air side is fan static plus wind, not zero.

Where this bites hardest is heat recovery. A bypass damper leaking across closed blades short-circuits treated air around the exchanger, and the de-rating lands directly on the temperature efficiency you declared. The same applies to the outdoor-air damper during full recirculation in a night purge or free-cooling sequence: every cubic metre that leaks in at −8 °C is a cubic metre the heating coil was not sized for.

Actuator sizing is an area calculation, not a catalogue guess

Actuator torque scales with damper face area and with how hard the blades have to be squeezed to close. Manufacturers publish a torque density in Nm per m² of damper, and it is not one number: a plain aluminium blade set without seals sits at the bottom of the published range, and a low-leakage damper with blade edge seals and side seals sits at the top — the seals that buy you the EN 1751 class are precisely what raises the torque.

So the sizing is: torque density × damper area × a margin, then check three things the torque figure does not tell you.

• Runtime. A smoke or fire sequence, a free-cooling changeover and a slow modulating mixing loop want very different stroke times. A 150 s actuator in a fast changeover is a control problem the commissioning engineer will blame on the sequence.
• Fail position. Spring return costs torque and money; specify it where a power failure must close the outdoor air, not everywhere by reflex.
• Blade linkage. A large damper split into coupled sections needs either multiple actuators or a jackshaft rated for the total — oversizing one actuator does not fix a linkage that twists.

The pressure drop nobody budgets

Fully open, an AHU damper is not free. Published figures for opposed-blade dampers at typical AHU face velocities of 2–3 m/s land in the low tens of pascals, and a mixing section carries two of them in the air path plus the turbulence they generate. That turbulence is a second, hidden cost: air leaving a mixing box is not uniform, so the filter downstream loads unevenly and the coil behind it sees a velocity profile nothing like the flat one its rating assumes.

Ten pascals here and twenty there is exactly the arithmetic that moves an AHU across an ECP-05-2026 class boundary, because the class is computed from internal pressure drop through the reference fan power — the same mechanism described in the duct connection article, where 50 Pa goes missing at the apertures. Internal pressure drop is a sum over every component, and a component omitted from the sum is a declared SFP that the unit will not reproduce on site.

Three practical consequences for anyone issuing a selection:

1. Quote every damper's open pressure drop at the design flow, not "negligible".
2. Quote the EN 1751 blade class and casing class with the test pressure.
3. Keep the mixing-section face velocity down — the dampers, the turbulence and the casing class are all working on the same square metres.

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